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Q. symmetrica, 103 µm long – stacked image

 

Quadrulella symmetrica (Wallich, 1863) Kosakyan et al., 2016

Diagnosis: Test ovoid or pyriform, with a rounded posterior end, laterally compressed towards the pseudostome. Test colourless, composed of square plates, regularly arranged in rows. The plates are smaller near the aperture (4–5 µm), then gradually larger (reaching up to 10–12 µm) towards the posterior end of the test. Pseudostome often curved and bordered by a thin organic lip.

Dimensions: Test length: 72–85 µm, breadth: 40–46 µm. Pseudostome: 20–23 µm wide.

Ecology: Q. symmetrica is found in wet mosses (Sphagnum or other), water streams, forest litter and soil, from all continents, except Antarctica.

Remarks: Morphologically very similar to Q. variabilis, from which it differs by the dimension of the test and the size of scale plates (L = 72–85 µm, maximum plate size 10–12 µm in Q. symmetrica versus L = 66–69 µm, maximum scale size 7–9 µm). It can be discriminated from Q. madibai, which has plates of similar size, based on its less slender and elongated test (L/ B ratio is 2.0–2.3 in Q. madibai versus 1.7–1.9 in Q. symmetrica). Moreover, the general outline of the test in Q. madibai is globally more tubular and does not present a distinct neck. Molecular data clearly separate these two species (sequence divergence up to 10%).

Detailed morphological and molecular observations are needed to clarify the true position of all described forms and their variability.

All these shells (76-147 µm long) were found in a sample from an oligotrophic stream in the Netherlands. The three larger tests belong to Q. variabilis (longicollis), the other ones are from Q. symmetrica. These records suggest rather a species complex than distinct intraspecific diversity. Probably they all belong to the same population.
Quadrulella symmetrica, 153 µm long – 3000 m. Mount Rainier, Washington USA
Quadrulella cf symmetrica, 78 µm long – stacked image
Quadrulella symmetrica, 84 µm, lateral view
Quadrulella symmetrica, 84 µm
Quadrulella cf longicollis – Austria, Tirol
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